An antigen shared by numerous species of a bacterial family may have biologic and clinical significance because (1) it may be used as indicator for the quantitation of the immune response, (2) it may serve as immunogen for immunization against infection by several potential pathogens, and (3) it may function as a pathogenic factor in immunologic injury. The present study is concerned with several common antigens of Enterobacteriaceae, including the common enterobacterial antigen, R core lipopolysaccharide, and lipoprotein. The cellular immune response to the antigens of guinea pigs, mice, and rabbits will be determined using skin tests and H3-thymidine incorporation of lymphocytes, and compared to the serum antibody response. The humoral immune response of animals immunized orally with bacteria producing common antigens will be investigated. The immunized animals will be studied pathologically also to determine whether immunization produces immunologic injury. The comparative study on the endotoxin-detoxifying capacity of sera from various animal species and their inhibitory effect on antigen-cell membrane interaction will be concluded. Because of the possible practical implications of the effects of attapulgite and other adsorbing drugs on toxicity of Vibrio cholerae and Escherichia coli enterotoxins will be continued. BIBLIOGRAPHIC REFERENCES: Whang, H.Y., Mayer, H., Schmidt, G. and Neter, E.: Strain-related differences in immunosuppressive effects of Enterobacteriaceae and their lipopolysaccharides on antibody production of rabbits to enterobacterial common antigen. Infect. Immun. 13: 1074-1079, April 1976. Schmidt, G., Mannel, D., Mayer, H., Whang, H. Y. and Neter, E.: The role of a LPS gene for immunogenicity of the enterobacterial common antigen. J. Bact. 126: 579-586, May 1976.